Ask about cantharadin (sp?). It is a treatment they apply with the tip of a toothpick (basically)... it is extracted from something from a beetle (some people have referred to it as bug juice, lol) combined with a prescription strength wart remover.
Our derm. applied this to the few molluscom that my daughter had, and it was a complete success.
http://en.wikipedia.org/wiki/Cantharidin
Your derm may not want to use it, simply because of the location of the molluscom your child has. My daughter's were on her neck and legs. Not on her bottom. This matters, because the treatment causes the wart to become extremely irritated, blister, and then their body's immune system finishes it off. Somehow the irritation signals the body's immune system "hey! this thing here is a problem!".
The treatment is applied (just a TINY dot of it on each wart), covered with a bandaid for about 3 hours, then you remove the bandage (at home) and wash gently but thoroughly with soap and water. Within a day or so it will become irritated and blister (and needs to stay covered with a fresh bandaid) and then after a few weeks (I think it took about 3 weeks for my daughter) it is gone.
But you sit on your bottom/backs of your thighs, so it might be too irritating to use for that location. Can't hurt to ask your derm. though.